The present invention relates to a charging device including a charge roller made up of a metallic core, an elastic member covering the core and films wrapped around the opposite end portions of the elastic member and contacting a body to be charged, an image forming apparatus including the charging device, an image carrier unit including the charging device, and a charging roller facing the surface of a body to be charged and applied with a voltage.
It is a common practice with a copier, printer, facsimile apparatus or similar electrophotographic image forming apparatus to uniformly charge the surface of a photoconductive element or image carrier, which is a body to be charged, before forming a latent image thereon. Corona discharge is one of conventional methods for charging the photoconductive element. A corona discharger includes a housing shielded by a metallic plate and a charge wire formed of, e.g., tungsten or nickel. The charge wire is positioned at the center of the housing in close proximity to the photoconductive element. A DC voltage or an AC-biased DC voltage is applied between the charge wire and the photoconductive element, so that the resulting corona discharge charges the surface of the photoconductive element.
The corona discharger, however, has a problem that the high voltage applied to the charge wire produces ozone, nitrogen oxides (NOx) and so forth, which are undesirable from the environment standpoint. Further, such products ascribable to discharge form a film of nitric acid or nitrate on the surface of the photoconductive element, adversely effecting image formation.
Today, a contact type charging device that produces a minimum of ozone and needs a minimum of power is replacing a corona discharger. A contact type charging device includes a conductive charging member implemented as a roller, a brush or an elastic blade. A voltage is applied between the charging member and the photoconductive element, which contact each other, in order to charge the surface of the photoconductive element.
The charging member implemented as a roller, for example, is made up of a metallic core and an elastic layer covering the core and formed of conductive rubber. This brings about a problem that when the contact type charging device is left unused over a long period of time with the roller being pressed against the photoconductive element, a plasticizer and other substances contained in the elastic layer ooze out and deposit on and contaminate the surface of the photoconductive element. Another problem with the contact type charging device is that when the charging member charges the photoconductive element in contact therewith, toner left on the photoconductive element after image transfer is likely to deposit on the charging member and lower the ability of the charging member.
In light of the above, Japanese Patent Laid-Open Publication Nos. 3-240076, 4-360167 and 5-107871, for example, disclose non-contact type charging devices. The non-contact type discharging devices each include a charging member implemented as a charge roller. Spacers or tapes, for example, are wrapped around the opposite end portions of the charge roller, forming annular projections larger in diameter than the intermediate portion of the roller. The projections space the portion of the charge roller other than the opposite end portions from a photoconductive element or image carrier. As a result, the portion of the charge roller corresponding to an image forming region does not contact the photoconductive element. This successfully solves the previously discussed problems particular to the contact type charging device.
Even the non-contact type charging device described above has the following problems left unsolved. In the charging device taught in, e.g., Laid-Open Publication No. 3-240076, the charge roller includes a surface layer formed of EPDM (ethylene-propylene terpolymer) or similar conductive rubber and spacer ring layers formed on the opposite end portions of the surface layer. Springs or similar biasing means press the portions of the charge roller outside of the spacer ring layers. In this condition, the portion of the charge roller other than the spacer ring layers is spaced from the surface of a photoconductive element.
In the above non-contact type charging device, only a DC voltage is usually applied to the charge roller. So long as a gap between the charge roller and the photoconductive element is smaller than preselected a preselected value (e.g. 20 xcexcm), a preselected charge potential is insured even if the above gap varies. However, once the gap exceeds the preselected value, the charge potential falls, i.e., decreases in absolute value in accordance with the gap. To compensate for the fall of the charge potential, an AC-biased DC voltage has customarily been applied to the charge roller. In this case, it is necessary to limit the voltage to a level that does not cause abnormal discharge to occur. The above gap should therefore be limited to a value that does not bring about abnormal discharge, limiting the thickness of the spacer rings.
Generally, an electrophotographic image forming apparatus includes an image transfer roller, a discharge roller, a developing roller and other charging rollers in addition to the charge roller. All of such charging rollers face an image carrier to be charged. When any one of the charging rollers constantly remains in contact with the image carrier in its portion other than opposite end portions, it is likely that residual toner and impurities are transferred from the image carrier to the roller and obstruct the expected function of the roller.
Technologies relating to the present invention are also disclosed in, e.g., Japanese Utility Model Laid-Open Publication No. 5-43156, Japanese Patent Laid-Open Publication Nos. 9-179444 and 9-258524, and Japanese Patent No. 2,740,998.
It is therefore an object of the present invention to prevent the intermediate portion of a charge roller from constantly contacting the surface of an image carrier even if films, or spacers, positioned on the opposite end portions of the roller are thin enough to obviate abnormal discharge around the films.
It is another object of the present invention is to prevent even a charging roller, which faces the surface of a body to be charged and is applied with a voltage, other than a charge roller from constantly contacting the above body in the intermediate portion thereof.
In accordance with the present invention, a charging device for charging the surface of a body to be charged includes a charge roller having a metallic core and an elastic member covering the surface of the core. Films are respectively wrapped around the opposite end portions of the charge roller such that the roller contacts the body to be charged via the films. A voltage is applied between the charge roller and the body to be charged. When the elastic member deforms due to compression ascribable to the contact of the charge roller with the body to be charged, the films deform along the outer periphery of the elastic member by a maximum amount, as measured in the radial direction of the charge roller, which is smaller than the thickness of the films.
Also, in accordance with the present invention, in an image forming apparatus including a charge roller, which has a metallic core and an elastic member covering the core, for charging the surface of a drum-like image carrier in response to a voltage applied between the charge roller and the image carrier to thereby allow a latent image to be optically formed on the above surface, films are respectively wrapped around the portions of the image carrier corresponding to the opposite end portions of the charge roller such that the opposite end portions of the charge roller respectively contact the films. The portions of the elastic member deformed due to compression ascribable to contact of the charge roller with the image carrier have a maximum deformation, as measured in the radial direction of the charge roller, which is smaller than the thickness of the films.